DNA is normally found as a double helix in which the Watson-Crick base pairs are held together by hydrogen bonding and the sugar-phosphate chains wind around each other in a right-handed manner. DNA directs the synthesis of proteins by serving as a template for polymerizing a strand of messenger RNA. This is accomplished by the enzyme RNA polymerase which separates the two strands of DNA and uses one of them as a template for RNA synthesis.
Recently it has been shown that the DNA double helix can exist not only in a right-handed B-DNA form but also in a somewhat less stable left-handed form, called Z-DNA due to the zig-zag organization of its sugar-phosphate chains. The base pairs in Z-DNA are "flipped over" relative to their orientation in B-DNA. This flipping process is associated with a rotation of alternate bases, usually purines, about their glycosyl bonds, so that they are in the syn conformation in Z-DNA, in contrast to the anti conformation found in B-DNA. Since purines [guanine (G) and adenine (A)] can adopt the syn conformation more easily than pyrimidines [cytosine (C) or thymine (T)], Z-DNA is favored by nucleotide sequences with alternations of pruines (Pu) and pyrimidines (Py). A number of factors determine whether Z-DNA is stable in a given piece of DNA. These factors are (1) the sequence of nucleotides, (2) the presence of negative supercoiling, (3) the binding of proteins which are specific for Z-DNA, (4) the methylation of CpG sequences at the carbon 5 position of cytosine, and (5) the presence of certain cations such as polyamines which stabilize Z-DNA.
Pertinent prior art are as follows: Structural aspects, of the left-handed Z-DNA double helix are outlined in the crystallographic study of Wang et al., Nature 282, 680-686, 1979. The identification of Z-DNA in natural DNA was first reported by Nordheim et al., Cell 31, 309-318, 1982, and the identification of Z-DNA in transcriptional enhancer segments was reported by Nordheim and Rich, Nature 303, 674-679 (1983).